Fix Orttraining Linux Lazy Tensor CI Pipeline (#21652)
### Description
Fix `Orttraining Linux Lazy Tensor CI Pipeline`
- Remove unused import of `torch.onnx._internal.exporter`, whose path is
changed in newer torch (pytorch/pytorch#132429).
- Move import of `register_custom_op_symbolic` from `torch.onnx` into
local function, which causes circular import when running `import
torch.onnx` (at least in the CI environment).
